This invention relates to vapor-liquid extractive distillation with a selective solvent.
To separate mixtures, for instance aromatics and non-aromatics by extractive distillation, a third component, a selective solvent can be added to the mixture to alter the relative volatility of the original constituents, thus improving the separation.
The utility of dialkyl sulfones in solvent extraction has been long recognized in the art. It has also been recognized that selective solvents may be blended with co-solvents including water. Water is particularly useful in many solvents in which it is at least partially miscible because it can reduce the boiling point of high boiling point solvents, reduce the freezing point of high freezing point solvents and increase the selectivity or separation factor of the components to be separated. This latter desirable characteristic is generally gained at a price, however, in the loss of solubility of the components to be separated in the selective solvent.
The improvement in separation factor gained by the use of water or any other co-solvent in a selective solvent system is quite unpredictable. For example, in U.S. Pat. No. 2,831,039, Table 2, separation factor for toluene-heptane in methyl ethyl sulfone is increased from 14.8 to 22.4 by the addition of 5 percent water while the same factor for methyl n-propyl sulfone is increased from 3.0 to only 3.03 for the addition of 5 percent water from the solvent.